Knuckle boom cranes have been extensively used because of their capability of reaching any desired point within the volume the tool attachment or the load application point describes. This is achieved by separate operation of the crane post, main boom, outer boom and telescopic part of the outer boom. Even in normal use, concurrent operation of all these functions requires an experienced, clear-thinking operator.
It has proved desirable to employ vehicle-borne knuckle boom cranes of the above-mentioned type also in mechanized tree felling and primarily in shrub cleaning, the felling or cleaning tool being attached at the load application point of the outer boom, since special machines cannot be used for economic reasons. Shrub cleaning means that a hydraulically driven flail is swung back and forth at a constant vertical level, both perpendicularly out from and along the direction of travel of the tractor. The vertical position of the flail above the ground should be approximately constant. This is achieved by constant changes of the mutual angular positions of the main boom and the outer boom, often in combination with telescopic extension of the outer boom. The cleaning operation must be effected rapidly since large areas are to be covered, and the suitable cleaning period is short. This means that the driver must run the vehicle and operate the outer boom while ascertaining that valuable plants and trees are spared and that the flail gets clear of stones and of the ground. This has proved troublesome with outer booms of the standard type. To facilitate this work, it is desirable to have a linear horizontal tool path which can be controlled by the driver with a single control instead of three. Such a path is obtained if the oil flows to the hydraulic cylinders of the main boom and the outer boom are positively controlled in such a manner that if the tip of the main boom moves upwards a predetermined vertical distance, the tool attachment of the outer boom is lowered the same distance. The tool thus remains on the same vertical level while simultaneously moving inwards towards the vehicle. Such a function is in principle achieved if the hydraulic fluid conduits of the outer boom are so connected that the oil which is pressed out from the lift cylinder when the main boom is raised, instead of returning to the oil tank, is conducted to the lowering side of the outer boom cylinder. However, this presupposes that the geometry and the cylinder dimensions agree with each other under certain given conditions. These are however determined primarily by the basic tasks to be performed by the outer boom, such as lifting timber. Only theoretical conditions would make it possible to use the outer boom both for timber transport and for shrub cleaning solely by switching the hydraulic fluid conduits.